18F-FDG uptake on PET helps predict outcome and response after treatment in unresectable thymic epithelial tumors

Positron emission tomography (PET) with fluorine-18-fluorodeoxyglucose (F-18-FDG) has been studied in thymic epithelial tumors. We evaluated the usefulness of F-18-FDG PET for monitoring after treatment in unresectable thymic epithelial tumors. Twelve patients with unresectable/metastatic thymic epithelial tumors underwent PET study with F-18-FDG before and after chemotherapy or radiotherapy. Response and survival were analyzed according to the ratio of the peak standardized uptake value (SUV) of the tumor to the mean SUV of the mediastinum (T/M ratio). Partial response (PR) evaluated by Response Evaluation Criteria in Solid Tumors (RECIST) was noted in 4 (33%) of 12 patients, and partial metabolic response (PMR) was observed in 6 (50%) of 12 patients. PR was observed in 4 of 6 patients with PMR. In 6 patients with any response, the T/M ratio at post-treatment was significantly lower than at baseline (p = 0.0017). In 6 patients with stable disease (SD), stable metabolic disease (SMD) was observed in 5 by use of F-18-FDG PET. No statistically significant difference of F-18-FDG uptake between at baseline and post-therapy was observed in 6 patients with SD (p = 0.4157) and SMD (p = 0.8419). Although the overall survival after treatment showed no statistically significant difference between PMR and SMD or progressive metabolic disease in the whole group of patients including thymoma, a statistically significant difference in the overall survival was observed between 5 patients with PMR and 5 patients with non-PMR (p = 0.0280). Our preliminary study suggests that F-18-FDG PET is useful for monitoring response and outcome after treatment in unresectable thymic epithelial tumors.